Floor plane reflection
As for the reason why it is happening, it is because the main direction of where your HDRI's light is coming from is from the bottom right corner of your image, and the floor is blocking that.



In order to fix that, if you go to the node editor, change the view to world shader and add a mapping and texture coordinate node for your HDRI, you can adjust the tilt of the HDRI.  If you connect the generate output of the Texture Coordinate node to the input of the mapping node and the output of the mapping node to the input of the Environment Texture, then you can control the angle of the HDRI.



You can use the rotation values in the Mapping node to adjust the HDRI's angle.